Signal transmission through optical fibers has become, or is becoming, the dominant signal transmission mode. The bandwidth characteristics of optical fibers, as well as their relative immunity to certain types of interference and contaminants make them the desirable transmission medium in high capacity trunk lines as well as in lower capacity feeder and distribution lines.
No matter what the intended end use may be, individual optical fibers generally are combined in an optical fiber cable which contains a plurality of such fibers, each of which is protected by at least one layer of coating material. In one configuration, the fibers are assembled into groups which are held together by binder ribbons or tubes to form a cable core. This is generally enclosed in a metallic or plastic tube or jacket which, in the latter case, often contains a strength member. In another configuration, the fibers are arrayed in ribbon form and the core tube contains one or more stacked ribbons.
Regardless of the cable configuration, it is usually necessary that the lengths of fiber cable be spliced at their ends to the ends of other cables, which entails splicing each of the individual fibers in a cable to a corresponding individual fiber in the second cable. To this end, there is provided a splice closure which usually comprises a protective case which contains at least one splice tray which, in turn, has a plurality of splice holders mounted thereon, into which the encased individual fiber splices are inserted and held. The cables are entrant into the case and generally are clamped to each end thereof to reduce the effects of tensile forces on the cables and on the splices. Sufficient amounts of fiber slack must be provided for in the case, such as, for example, half a meter of fiber length so that the individual fibers can be pulled clear of the case to effect the splice. The slack also serves the important function of absorbing tensile forces, thereby isolating the splices from such forces. Because of the delicate and brittle nature of individual glass fibers, they cannot be crimped or bent too sharply, i.e., bent to too small a radius of curvature which places restraints upon slack storage. Thus, there have been numerous arrangements in the prior art addressing the problem of fiber and slack storage, as exemplified by U.S. Pat. No. 5,097,529 of Cobb, et al.; U.S. Pat. No. 4,679,896 of Krafcik, et al.; and U.S. Pat. No. 4,332,435 of Post.
Inasmuch as, at the splice point, the cable itself is opened up and the base fibers are exposed, the only protection afforded the fibers is provided by the closure, which can provide only one or two layers of protection from the outside environment, the requirements therefor are more stringent than for the cable, which normally provides several layers of protection. The closure must anchor the cables stored therein, and it must be capable of withstanding torsional and axial loads transmitted by the cable to the closure so that the splices are protected from these loads. The closure must also seal the inner and outer sheaths of the cables and maintain the seal integrity under extreme environmental conditions. In addition, the closure must provide adequate fiber storage for slack fiber without damaging the fibers and without increasing signal attenuation. The closure preferably should be capable of storing any type of splice, such as, for example, discrete mechanical, discrete fusion or mass mechanical, or other types while reducing forces that tend to damage the splices. Additionally, the closure should provide adequate grounding and anchoring for the metallic strength members of the cable. The closure should also be capable of accepting high fiber count cables as well as those of low fiber count.
Typically, prior art splice closures are somewhat complex, difficult to assemble, are necessarily bulky, and, in use, difficult to access. As a consequence, they are not economical when used for splicing relatively low count fiber cables, such as, for example, drop cables or CATV applications. Also, when used for low fiber count cables, the bulkiness of the closure makes it difficult to provide adequate storage room, without sacrificing accessibility. This problem of size has heretofore been addressed by simply using a large closure designed primarily for high capacity use, where feasible, or by designing special, smaller closures for low capacity use, which cannot carry or contain large numbers of fibers and splices.
In order to insure protection of the splices from moisture, it is current practice to form the closure out of two mating halves, with a grommet therebetween, and clamp them together. Cable entry is through openings in the grommet, which are usually supplied with inserts which seal the cable and in turn are sealed by the grommet. Such a grommet and insert arrangement is shown, for example, in U.S. Pat. No. 5,472,160 of Burek, et al.. In that arrangement, the grommet, which is of a resilient material suitable for moisture sealing, has, at each end thereof, first and second seal members having bores therein for receiving grommet inserts, which, in turn have bores therein for receiving the cable. The seal members are preferably split longitudinally so that the grommet inserts, with cables extending therethrough, can be inserted in the seal members and be tightly embraced thereby. When the two halves of the housing are clamped together, the cable is tightly embraced, as are the seal members, so that a watertight seal is achieved.
In usage, it has been found that such a sealing arrangement can be vulnerable to a bending or flexing of the cable adjacent the entrance to or exit from the closure which can, in some instances, break the integrity of the seal. There have been various arrangements in the prior art for correcting this effect, one such arrangement being shown in U.S. Pat. No. 5,434,945 of Burek, et al. wherein the closure is encased in a protective shell which, after assembly, is filled with an encapsulant. Such an arrangement insures that the splice closure itself is virtually certain to be moisture proof. However, access to the splices is made more difficult by the presence of the encapsulant, which must be removed to permit such access. For high fiber count cables, limited access, while undesirable, does not necessarily pose too much of a problem. However, for low fiber count cables, where frequent access may often be required, such difficulty of access is undesirable.
Cables entrant into the enclosure are preferably, and in present day usage, almost always anchored to the splice enclosure itself, to guarantee a minimum of movement of the cable within the enclosure which could unduly stress the fibers and the fiber splices. One such anchoring means, in the form of a cable grip block, is shown in the aforementioned Burek, et al. U.S. Pat. No. 5,472,160. The grip block of that patent is capable of adapting to cables of different sizes, and also provides an anchor for the cable central strength member which is a usual component of loose tube type cables. The anchoring arrangement for the central strength member requires that the strength member or members be cut to a specific length and bent upward into a slot within the grip member. The strength members are maintained within the slots against tensile forces because of their rigidity and because they are bent at a right angle. Thus, they function to help maintain the cable against shifting or movement. While this arrangement functions well, the necessity of cutting the strength members to specific lengths and of bending and threading them into the slots adds another increment of assembly time to the splice closure system.
Beyond the cable grip block, the cable is opened up, i.e., the cable jacket and/or sheaths are removed to free the individual fibers so that splicing can be effected, commonly referred to as fiber breakout. It is generally necessary, or at least desirable, that sufficient amounts of fiber slack be provided within the case, or enclosure as discussed hereinbefore, so that the individual fibers can be pulled clear of the closure for preparation of the ends for splicing and for effecting the splice. For a multifiber cable there should be, within the closure, some arrangement for positioning and storing the slack and for keeping the fibers arranged in an orderly manner. Prior art arrangements that address the problem of both splice and fiber slack storage and organization are shown in the aforementioned Cobb, et al., Krafcik, et al., and of Post, as well as in U.S. Pat. No. 5,420,957 of Burek, et al. As can be seen in these patents, a splice tray is used to hold and protect the splices themselves by means of a splice holder as well as providing space for the slack fibers to be contained without the necessity of too sharp bends. For example, a splice tray adapted to be mounted within the closure, as shown in the Burek, et al. U.S. Pat. No. 5,420,957 comprises an elongated substantially rectangular container having circular hubs at each end about which the fiber is to be wound, the radius of the hubs being greater than the critical bend radius, and a splice holder located between the hubs for containing and holding the splices. Such a tray has become a widely used component of present splice closures, and, where large members of splices are to be contained, various arrangements for stacking the trays may be provided as shown, for example, in U.S. Pat. No. 5,481,639 of Cobb, et al. In most of the aforementioned arrangements, the closure is designed to hold one or more splice trays having a specific configuration, and usually is not adaptable to accommodating splices for different kinds of fiber configurations, e.g., individual fibers or fiber ribbons. More particularly, it is generally economically unfeasible to modify a given closure to accept more than one type of tray.
Most of the prior art arrangements, as evident from the foregoing discussion, emphasize adequate storage capacity and fiber protection, with space and ease of access being secondary considerations. Certain ones of the aforementioned patents, such as the Cobb, et al. patent, make size, access, and cost important considerations, at least for some situations. However, even greater reductions in size, complexity, and cost are needed, as well as a greater measure of versatility or adaptability coupled with a ready accessibility.